Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual User Manual
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Data Link Layer
Appendix E
E-2
When a station gains control of the Data Highway to transmit messages, it
has become a master station. All other station interface modules assume a
slave mode. This enables these stations to receive and acknowledge
messages sent to them. Basically, a station has three states of operation:
Transmitting Messages
Polling to Determine which Station Gets Mastership Next
Receiving Messages and Polls
Thus, each Data Highway station can transmit and receive both messages
and polling sequences.
Figure E.1 shows the change of states at a Data Highway station.
Figure E.1
State Transition at Master Station
11536
Received
Mastership
Polling
Sending
Received
Relinquished
Mastership
All Sent or Max. of 16 or
All High Priority Messages
Sent and Response to High
Priority Global Poll
Contention
Have
Messages to
Send and All
Stations Have
Been Polled
at Least Once
State
State
State